1. Field of the Invention
The present invention relates to a light beam scanner for deflecting a light beam to scan an image-carrying surface, and more particularly to a light beam scanner capable of scanning such an image-carrying surface highly accurately with a scanning light beam free of scanning pitch variations.
2. Description of the Prior Art
Various systems have been developed in recent years for reading and/or recording images with light beams. In such a system, the light beam emitted from a light beam source is reflected and, at the same time, deflected by a light beam deflector such as a rotating polygonal mirror to scan an image-carrying surface (main scanning) which is being fed at a constant speed (auxiliary scanning) in a direction normal to the direction in which the light beam is deflected. The light beam deflectors in the conventional light beam scanners are however susceptible to axial displacements with the result that the scanning light beam as deflected by the light beam deflector for scanning the image-carrying surface is liable to be distorted in the auxiliary scanning direction. Where the light beam deflector comprises a rotating polygonal mirror, it would be technically difficult to make the light reflecting surfaces of the mirror completely parallel to the axis of rotation of the mirror, and the scanning beam pitches are rendered different by such mirror surface irregularities. There have been proposed a variety of scanning arrangements which employ an anamorphic optical system between the light beam deflector and the image-carrying surface to be scanned in order to compensate for such mirror surface irregularities.
One example of the proposed scanning systems will be described with reference to FIGS. 3 and 4 of the accompanying drawings. A laser light beam 102 emitted from a laser beam source 101 is adjusted by a beam expander 103 into a light beam of an appropriate beam diameter, which then passes through a cylindrical lens 104 and falls on a light beam deflector 105 in the form of a rotating polygonal mirror as a linear image perpendicular to the axis of rotation thereof. The light beam 102 is reflected and deflected by the rotating polygonal mirror 105 when it rotates in the direction of the arrow m, and travels along a light beam path, which is viewed in a direction parallel to the axis of the rotating polygonal mirror 105 in FIG. 3 and in a direction normal to the axis of the rotating polygonal mirror 105 in FIG. 4.
As illustrated in FIG. 3, the light beam 102 reflected and deflected by the rotating polygonal mirror 105 impinges as a parallel beam upon a spherical lens 107 such as an f.theta. lens positioned on the light beam path. After having passed through the spherical lens 107, the light beam 102 converges on an image-carrying surface 108 spaced from the spherical lens 107 by the focal length f.sub.107 thereof. The image-carrying surface 108 is scanned in the direction of the arrow A in an area between a.sub.1 and a.sub.2 (main scanning) when the rotating polygonal mirror 105 rotates in the direction of the arrow m. Another cylindrical lens 106 is disposed between the spherical lens 107 and the rotating polygonal mirror 105 and extends longitudinally in the direction in which the main scanning takes place. The cylindrical lens 106 is spaced from the rotating polygonal mirror 105 by its own focal length f.sub.106. The cylindrical lens 106 serves to refract the light beam 102 only in a direction (auxiliary scanning direction) normal to the main scanning direction. The light beam 102 appears in FIG. 3 to pass simply through the cylindrical lens 106. The spherical lens 107 and the cylindrical lens 106 jointly constitute an anamorphic optical system. The spherical lens 107 is shown to be a so-called scanning lens. Instead of the spherical lens 107, an axially symmetric aspherical lens may be employed as such a scanning lens.
The rotating polygonal mirror 105 is apt to suffer reflecting surface irregularities as described above. FIG. 4 shows a system for compensating for such mirror surface irregularities through the anamorphic optical system.
The light beam 102 reflected by the rotating polygonal mirror 105 falls on the cylindrical lens 106, which converts the light beam 102 into a parallel beam because the cylindrical lens 106 is spaced from the rotating polygonal mirror 105 by its own focal length f.sub.106. The parallel light beam 102 then travels through the spherical lens 107 and converges on the image-carrying surface 108 that is spaced from the spherical lens 107 by the focal length f.sub.107 thereof. In the absence of reflecting surface irregularities, i.e., if the mirror surfaces are parallel to the axis of rotation of the mirror 105, the light beam 102 goes through the light beam path indicated by the solid lines. If, however, a reflecting surface suffers an irregularity, e.g., a reflecting surface 105a is displaced to a position 105a', then the light beam path is also displaced to a position indicated by the dot-and-dash lines. With the illustrated anamorphic optical system, the light beam traveling along the light beam path indicated by the solid lines and the light beam traveling along the light beam path indicated by the dot-and-dash lines originate from the same point on the reflecting surface 105a, and are converted by the cylindrical lens 106 equally into parallel beams which fall on the spherical lens 107. Therefore, the light beam indicated by the solid lines and the light beam indicated by the dot-and-dash lines converge on the image-carrying surface 108 at the same point a.sub.3. As a result, deviation of the light beam path in the vertical direction in FIG. 4 arising from a reflecting mirror surface irregularity, for example, can be compensated for.
The rotating polygonal mirror is however disadvantageous in that as the mirror rotates, the reflecting surfaces thereof are successively moved around the axis of the mirror into and out of the position facing the cylindrical lens 106, thereby causing the light beam coming from a fixed direction to be reflected from different positions on each of the reflecting surfaces. This is undesirable since the length of the light beam path is varied, preventing the light beam from converging on the image-carrying surface at all times.
The above problem will be described in greater detail with reference to FIG. 5. It is assumed that the light beam 102 falls on the rotating polygonal mirror 105 in a constant direction, and the image-carrying surface is scanned by the light beam in the main scanning direction A. As the rotating polygonal mirror 105 rotates in the direction of the arrow m, the length of the light beam path from the light beam source up to the rotating polygonal mirror 105 is continuously varied as is apparent from comparison in FIG. 5 between the solid-line position of the rotating polygonal mirror 105 and the dot-and-dash-line position of the same. Therefore, even if the light beam 102 is reflected as a circular beam spot by a beam reflecting position when the rotating polygonal mirror 105 is in the solid-line position, the light beam 102 will be reflected as an elliptical beam spot from a different beam reflecting position when the rotating polygonal mirror 105 is in the dot-and-dash-line position. Furthermore, the beam reflecting position on the rotating polygonal mirror 105 in the solid-line position and the beam reflecting position on the rotating polygonal mirror 105 in the dot-and-dash-line position are spaced from the image-carrying surface by different distances, the difference between them being indicated by S. In case the beam reflecting position of the rotating polygonal mirror 105 and the image-carrying surface being scanned are to be in a conjugate relationship when the rotating polygonal mirror 105 is in the solid-line position, no such conjugate relationship is achieved as long as the rotating polygonal mirror 105 is not in the solid-line position. The light beam reflected from any beam reflecting position which is not in a conjugate relationship to the image-carrying surface does not converge completely on the image-carrying surface and hence is out of focus thereon. The foregoing effect which the continuous angular movement of the reflecting surfaces of the mirror 105 has on the light beam position on the reflecting surfaces and the light beam focus on the image-carrying surface can be reduced to a certain extent by positioning the cylindrical lens 106 between the spherical lens 107 and the image-carrying surface 108 as closely to the image-carrying surface 108 as possible. However, such an arrangement not only fails to solve the above problem completely, but also is practically infeasible inasmuch as it requires an elongate and hence expensive cylindrical lens having a length close to the width or interval to be scanned.